(1) Field of the Invention
This invention generally relates to a device for towing long multiple line arrays.
More particularly, the invention relates to a device for towing long multiple linear arrays with a streamlined composite tow cable, with the composite tow cable having direct mechanical and electrical connections to each towed array.
The array towing arrangement and bridle configuration present minimal drag to the tow platform and consequently the proposed arrangement can be towed at greater speeds than other more traditional towing arrangements. The reduced drag enables higher tow speed and in turn more rapid coverage of large areas of the ocean floor.
(2) Description of the Prior Art
The current art for undersea oil exploration is guided by seismic analysis of the ocean floor. Large surface ships 90 tow multiple linear arrays 92 equipped with low frequency acoustic transmitters and receivers (see FIG. 1). An array catenary 94 trails from the tow platform/ship 90 and is supported by interconnecting cables 96 at a location of a pair of depressors 98, one at each of the outer spread of the interconnecting cables 96 as shown. These arrays 92 send powerful seismic impulses into the sea floor. The received echo provides information about the geological make-up of the ocean floor and also provides indications as to the location of natural resources. The individual linear arrays 92 can be many miles long and the bridle holding many arrays can be several miles wide. The combined drag of the required depressors 98, interconnecting cables 96, and array catenaries 94 can be enormous. This drag limits the operational speed of the tow platform 90 and consequently the rate at which the ocean bottom can be explored. Because the tow platform 90 must be extremely powerful to tow the large arrays, the operational costs are very high. An improved tow configuration which reduces the total array drag and thus enables more rapid seismic surveys is needed in the art. Correspondingly, a reduction in the required tow platform scale could save the ocean exploration industry time and resources.
The following patents, for example, disclose towing cables and towed arrays, but do not disclose an asymmetric tow system with a streamlined composite cable having both an electrical and mechanical connection to each of plural towed arrays.
U.S. Pat. No. 4,317,185 to Thigpen et al.;
U.S. Pat. No. 5,089,668 to Harvey;
U.S. Pat. No. 5,274,603 to Zibilich, Jr. et al.;
U.S. Pat. No. 5,408,947 to Curto et al.;
U.S. Pat. No. 5,532,975 to Elholm; and
U.S. Pat. No. 5,913,280 to Nielsen et al.
Specifically, Thigpen et al. discloses a towing link consisting of a spaced-apart head and tailpieces. The headpiece includes a towing eye and a pair of lugs for receiving the stress members of a streamer and a lead-in cable. The tailpiece defines a pair of bores through which the stress member and electrical conductors of the two cables are inserted and sealed. A pair of mating connector plugs is provided to interconnect corresponding conductors of the two cables. The assembly is enclosed in a watertight plastic boot.
The patent to Harvey discloses a towed streamer having a buoyant core, a data bearer layer surrounding the buoyant core, an inner jacket layer formed of a resilient material surrounding the data bearer layer, and an outer jacket layer surrounding the inner jacket layer, wherein a series of longitudinally positioned strength members are embedded in the inner jacket layer to extend along the streamer to transmit tension along the streamer while the streamer is held in the inner jacket layer.
Zibilich, Jr. et al. discloses a marine seismic cable section with stress members and an internal wiring located within the cable section in a manner which reduces internal bending stresses upon the cable section when wound upon a storage reel. Stress members are at least near a horizontal plane passing through and at approximately equal distances from the center line of the cable section. Internal wiring is located in a vertical crisscrossing pattern down the length of the cable section. In one embodiment, the cable section contains an elastomeric filler material to retain and support stress members, internal wiring and other internal components at their desired location and to prevent damage to internal wiring and components due to external stress when the cable section is wound on a cable reel. Stress relief sections are also provided to further reduce bending stress within the seismic cable section in other embodiments. Additional embodiments to the cable section provide for adapting the cable section to a bottom cable which is laid on and couples with the sea floor to detect all three vectorial components of particle motion resulting from p- and s-waves and to detect p-waves with a hydrophone. Other embodiments provide for sealing and termination methods which are compatible with use of elastomer as a filler material for connecting detectors disposed within the cable section to internal wiring.
Curto et al. discloses a method and apparatus in marine seismic surveying for towing an optical-electrical towing cable (lead in) and seismic array (streamer cable) at a perpendicular distance from the centerline of the towing vessel using a short, flexible adapter cable section which optically and electrically connects the lead in to the streamer cable and which attaches to pivoting arms of a removable towing bracket which carries the bending loads.
The patent to Elholm discloses a positioning device for seismic equipment which is towed by a seismic vessel and is designed with a body part which is equipped with wings and rudders. For the control of wings and rudders, control means, preferably hydraulic or electrical means are used. The device further comprises a control unit for processing of signals which preferably operate exclusively on the basis of information from the vessel or the ship, instruments for use in the positioning of the device and a communication system for the communication between the vessel and the device and vice versa, preferably electrical, acoustic or optical. It is further equipped with attachment devices of one or more cables and floats, which are preferably provided at the front of the device, and preferably in the vicinity of the wing's attachment point to the device's body part as well as a power supply system.
Nielsen et al. discloses methods, systems, and towing bridles, provided to increase spread width, streamer separation, and number in marine seismic data acquisition. The invention is applicable for towing seismic equipment behind a marine seismic data equipment handling vessel along a data acquisition path, the system comprising: a deflector attached to a deflector line; an equipment handling vessel attached to the deflector; a deflector line pulling vessel attached to the deflector line; and the marine seismic data equipment attached to the deflector line.
It should be understood that the present invention would in fact enhance the functionality of the above patents by providing a streamlined composite tow cable with both mechanical and electrical components therein for mechanically supporting and electrically connecting the towed arrays to a tow platform.